Need to make sure your phone and tablet both turn on at the airport when you fly off on holiday? If you want to take them on board, security may now insist on seeing then work.

Or do you just need to give your battery a boost from time to time to get through a long day? Whether it's GPS navigation, an addiction to 2048 or an older device with a flagging battery, an external battery can keep you going.

But you have to choose how big you want your battery to be, both in size and capacity. If you want more amp hours or enough voltage to charge a tablet instead of just a phone, you usually have to lug a larger battery around with you.

Not with Idapt's clever modular battery. You can buy a couple of smaller batteries, charge them up individually or together and either carry just one — or snap then together to get a bigger power source. You can even add a $25 solar charger to keep them charged when you're out and about or a $10 wireless charging receiver to charge it from any Qi charging plate. And IDAPT is even making a hand crank for recharging.

Each Modulo is about two inches by 3.5 inches and half an inch thick (the size of a stack of credit cards). Three LEDs on the front light up when the Modulo is charging and you can press the (rather stiff) button to see how much charge you have left as one, two, or three LEDs light up.

The chamfered end and the grooves at the side let you slide one Modulo on top of another

One end of the Modulo has a slight bevel and there are grooves along the side of the base; that means you can slide one Modulo on top of another and it fits securely, making a thicker block. You can stack up to five of them and they'll all charge off one cable (or one wireless charging plate); a full charge takes an hour or two.

Stack two Modulo batteries together to get a bigger charge for a tablet

You only get a short (4-inch) USB to micro USB cable in the box — use that to charge the Modulo, then turn it round and plug it into the USB port and you can use it to charge a phone or tablet. If you have a gadget that needs a special cable, you can just plug that into the USB port — you don't get any extra tips, cables or adapters in the box, but many devices use micro USB for power now.

It's when you start stacking Modulos that it gets interesting. The power output is 1.2A to 2.4A; with one Modulo you can give a full charge to your smartphone or a part charge to a tablet. In 10 minutes, a partially-charged Modulo took a completely flat Dell Venue Pro 8 up to five percent charge — enough to check a document, send an email or prove to airport security that it's a working device.

But if you really want to charge a tablet, slide two Modulos together. We charged a completely flat Lumia 1520, which has a tablet-sized 3400mAh battery, from two fully charged Modulos. It took two hours to get a 50 percent charge; that left one Modulo drained and the other with two bars of power showing. You could also charge two smartphones at once from two Modulos; with three you have 3.6A of power so you could charge three phones, or a phone and a tablet at once, and so on.

Some external batteries offer faster charging. Innergie's PocketCell and PowerDuo pump out more power more quickly because they use laptop-grade batteries, but they're much pricier and you'll definitely feel the heat when you use them — the Modulo got a little warm but it would be comfortable in your pocket. And at $25 to $30 each, they're both cheap and functional.

Originally produced as a Kickstarter offer, the Modulo range will be on sale soon (you can pre-order it now). We looked at two of the early units; now you have the option of a slightly higher battery capacity (2500mAh each instead of 2100mAh) for another $5, so you'll get a little more charge than in our tests. They come with a striking fuchsia panel on the white case, but there are bright blue and black stickers in the box if you want a different colour — or you want to be able to tell your Modulos apart.

Minimal packaging and no extras keeps the price of the Modulo low; we've already stuck the Smart Attach onto the Lumia

You also get a 'Smart Attach' plate you can stick on to your phone; this fits into the grooves of the Modulo so you can slide the battery on while you're charging and have it neatly held in place instead of holding them together in your hand. The short cable is ideal here. If you've ever broken a cable by putting the phone and the battery that's charging it in a bag or pocket (micro USB connectors can be fragile), this could come in particularly handy.

The plate isn't too obtrusive when it's stuck to the back of your phone and you can peel it off again; rather than glue, it uses the same renewable sticky surface as car mounts so it doesn’t leave sticky mark on the back of your phone. And if it stops sticking you can get the grip back by washing the two sticky strips with soap and water. We also liked the minimal but stylish cardboard packaging.

There are more external batteries on the market than we can keep track of, but the IDAPT Modulo is a clever design because you can stack them up and break them apart depending on what you need to power. You don't have to always carry a heavy battery if you usually only need to top up your phone, but you can easily expand it when you want to charge a tablet.

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